For many years, steam has been reliably used for electric power generation, various process needs, or both. The basic steam generation process operates on the Rankine cycle. Water is fed to a boiler where it is evaporated to generate high pressure steam. Coal is a common fuel for firing steam boilers, although other fuels may be used. The steam is expanded in a turbine which drives a generator. The steam may be condensed and returned to the cycle, or passed out to be used in industrial processes. Many modifications of this basic steam cycle are possible.
The steam cycle has also been used in combination with a gas-turbine cycle for various purposes. In the basic gas-turbine cycle, following the Brayton or Joule cycle, a working fluid, typically air, is compressed, heated, and then expanded in a turbine to drive a generator. The compressor is generally mounted on the same shaft with the turbine so that the turbine is used to drive the compressor as well. A variety of fuels, such as natural gas, kerosene, or diesel fuels, may be burned in a gas turbine combustor to heat the working fluid. The working fluid is typically air, which is also a source of oxygen for combustion processes, mixed with the combustion products from the fuel. Thus, the fuel must be relatively clean to avoid damaging the turbine blades.
Gas turbines typically exhaust gas at temperatures which are greater than the temperatures needed to generate steam. Thus, in the combined steam-gas turbine cycle, the exhaust from the gas turbine may be introduced into a heat recovery boiler to generate the steam for the steam turbine. Supplementary fuels may be added to the heat recovery boiler in some cases. The gas turbine cycle may also be used as a supplement during peak power loads or start-up of the steam cycle.
A drawback to combustion processes which burn fossil fuels, such as coal, fuel oil, or natural gas, is the generation of harmful pollutants. Oxides of nitrogen or NO.sub.x, NO and NO.sub.2, are such pollutants which are linked to acid rain and the production of smog. NO.sub.x emissions from coal combustion originate mainly from the nitrogen compounds organically bound in the fuel, whereas NO.sub.x emissions formed in natural gas flames are due to the fixation of atmospheric nitrogen at high temperatures and oxidizing atmospheres (thermal NO.sub.x).
Various techniques for reducing NO.sub.x emissions are in practice. Thermal NO.sub.x formation is temperature dependent and occurs rapidly at temperatures greater than 1800 K. Thus, controlling the temperature in the combustion zone to remain below approximately 1800 K. is effective in controlling thermal NO.sub.x formation.
Staged combustion, in which a first fuel-rich combustion zone is followed by a second fuel-lean combustion zone, is effective in reducing NO.sub.x emissions. See, for example, U.S. Pat. No. 4,845,940 to Beer. In the fuel-rich zone, less than the stoichiometric amount of combustion air is supplied, resulting in less available oxygen to react with the nitrogen. Thus, the nitrogen is more likely to convert to N.sub.2 rather than NO.sub.x. The rest of the combustion air is introduced downstream from the first combustion zone in a fuel-lean zone to complete the combustion. Also, staging the supply of air into the combustion zone tends to stabilize the temperature, which minimizes thermal NO.sub.x formation.
Another NO.sub.x emission control method which has been demonstrated to be technically and economically feasible is the NO.sub.x "reburn" method. This method is based on the high temperature reactions between NO.sub.x and hydrocarbon fragments in fuel rich atmospheres. A hydrocarbon fuel such as natural gas is injected into hot NO.sub.x -bearing combustion products in such a way that the fuel-air mixture ratio becomes fuel rich. A fraction of the NO.sub.x in the combustion products reacts through a series of reactions with the hydrocarbon fragments to give molecular nitrogen, N.sub.2. Following the NO.sub.x reducing reactions more air has to be injected to achieve complete combustion. This technique may be applied by recirculating flue gas from a furnace or boiler back into a natural gas or oil fired burner from which the gas was originally generated.